The present invention relates to a process of forming electrodes and interconnections on silicon semi-conducor devices, and more particularly to a process of rendering less resistive thin layers of polycrystalline silicon or thin impurity-doped layers which are employed as interconnections in high-speed, high-density large scale integrated circuits, and of forming thermally resistant ohmic electrodes for the impurity-doped layers.
To meet the demands for higher density and higher-speed operation, great efforts have continuously been made to reduce the three-dimensional size of MOS LSI devices according to the scaling law. The trend for smaller devices has led to reduced thicknesses of gate electrode interconnections and impurity-doped layers which serve as interconnections formed on silicon substrates. The thinner interconnection layers have, however, resulted in increased sheet resistivity, which has prevented devices from operating at higher speeds. To solve the problem, there have been proposed a process of covering the surface of an impurity-doped layer with a metal silicide having a low electric resistance, and a gate electrode construction formed of a high-melting-point metal or its silicide rather than polycrystalline silicon, or of polycrystalline silicon covered with a thin layer of metal silicide. The metal silicide used is required to be lower in resistance than the impurity-doped layer, to be thermally resistant up to at least about 1,000 degrees Celsius, and to be formed as a layer uniformly self-aligned with respect to the impurity doped layer, contact holes and pattern of interconnections. It is also desirable that the area where the metal silicide is held in contact with a layer of aluminum be thermally resistant up to at least about 500 degrees Celsius. One known process of forming a thin film of metal silicide on a silicon substrate or on a thin film of silicon comprises the steps of depositing a metal film on silicon to provide a metal/silicon construction, and then merely annealing the composite construction to cause silicide forming reaction to take place at an interface. Since silicide forming reaction between precious metal such as platinum and silicon can easily proceed during annealing at a relatively low temperature of less than 500 degrees Celsius, the above fabrication process has been studied for the production of a silicide of precious metal for the foregoing applications.
However, as reported by A. K. Sinha et al. in Journal of Applied Physics, Vol. 43, No. 9, Pages 3637-3643, 1972, a precious metal silicide has a relatively low melting point and hence is poor in thermal resistance. The annealing step for the silicide formed therefore needs to be effected at a temperature lower than approximately 800 degrees Celsius. Furthermore, P. C. Parekh et al. have pointed out in Solid State Electronics, Vol. 19, Pages 493-494, 1976, that a precious metal silicide starts reacting with aluminum at a relatively low temperature lower than 350 degrees Celsius. These shortcomings could be eliminated by employing a metal silicide having a high melting point such as a silicide of molybdenum which can withstand annealing at about 1,000 degrees Celsius and which will start reacting with aluminum at a relatively high temperature about 500 degrees Celsius. However, where a silicon substrate is doped at its surface with impurities such as arsenic or phosphorus at a high density, a metal having a high melting point is often liable to fail to get silicide forming reaction even if annealed at a high temperature in the vicinity of 1,000 degrees Celsius. Therefore, the silicide forming reaction using a high-melting-point metal under the above condition sometimes fails to progress. Even a silicide which has successfully been formed through such reaction is extremely poor in uniformity and flatness of the surface and an interface between the silicide and silicon, and hence finds difficulty in application to devices. The silicide thus formed also fails to become self-aligned with contact holes, as reported by R. W. Bower et al. in Applied Physics Letters, Vol. 20, No. 9, Pages 359-361, 1972, and by A. K. Shinha et al. in Journal of Electrochemical Society, Vol. 120, No. 12, Pages 1767-1771, 1972.
More specifically, as shown in FIG. 1 of the accompanying drawings, a silicide 14 of metal having a high melting point is formed by forming an opening in an insulating film 12 formed on the surface of a silicon substrate 11, depositing a metal having a high melting point over the entire surface to form a film/3, and annealing the deposited film 13 at 625.degree. C. in a vacuum. Such a silicide layer 14, however, is not confined within the contact hole, but spreads out onto the insulating film around the contact hole.
As an alternative to the above annealing process, there has been proposed in recent years a process of forming a metal silicide by implanting ions into a metal/silicon construction under the condition that its interface is mixed together. S. W. Chiang et al. have reported in Journal of Applied Physics, Vol. 52, No. 6, Pages 4,027-4,032, June 1981, that phosphorus ions are implanted into a molybdenum/silicon structure to form a silicide of molybdenum. The article has described that such a silicidizing reaction is more repeatable, and the resultant silicide remains uniform upon annealing at 850 degrees Celsius. However, the uniformity of the silicide becomes extremely poor when subjected to annealing at a higher temperature, such as 1,000 degrees Celsius. Such a non-uniform silicide is by no means suited for rendering less resistive thin impurity-doped layers in small-sized MOS LSI. This paper is silent as to self-alignment of the silicide which is required for high-density circuit construction, that is, whether the silicide spreads out of a contact hole.
S. W. Chiang et al. used a structure as shown in FIG. 2 for their experiments. The structure includes a substrate 21 of n-type silicon having a resistivity of 1-4 .OMEGA.cm, and a silicon oxide film 22 having a thickness of 6,000 .ANG. formed on the silicon substrate 21. After the oxidized-silicon film 22 has been formed on the silicon substrate 21, a large circular hole having a diameter of 0.65 mm is defined in the film 22. Then, a film 23 which is 1,000 .ANG. thick is deposited on the silicon oxide film 22 entirely over its surface by way of DC magnetron sputtering, the film 23 being made of molybdenum. The molybdenum film 23 is then patterned by photoetching to a 0.76 mm diameter circular form which is concentrical with the circular hole in the silicon oxide film 22. Thereafter, phosphorus ions are implanted for mixing the interface between the molybdenum and silicon layers, and annealed in an H.sub.2 gas atmosphere at a temperature of 850 degrees Celsius or more. The present inventors however have experimentally found that under the above annealing condition, the silicide formed is not self-aligned with the circular hole along its edge and has a margin extending over the silicon oxide film around the circular hole. The report presented by S. W. Chiang et al. contained no description about behavior of the silicidizing reaction at the edge of the circular hole. Considering the fact that the film 23 of molybdenum is positioned by photoetching with respect to the circular hole so that the film 23 is patterned to have its margin extending over the film 22 around the circular hole, S. W. Chiang et al. did not appear to intend to cause the silicide to be self-aligned with the circular hole. The circular opening defined in the silicon oxide film 22 is so roughly and largely sized that it will not match a highly compact and complex LSI pattern, and does not appear to be well calculated for application to actual small-sized LSI devices.